1. Related Field
The present invention relates to an optical fibre interrogation system and in particular an optical fibre interrogation system for condition monitoring.
2. Description of Related Art
Interrogation schemes for the most common type of optical fibre transducers, fibre Bragg gratings (FBGs), require precise detection and location of the FBG centre wavelength. This can be realised using a number of methods, but the most common uses a combination of a broadband light source, tunable filter and photodetector, so that wavelength division multiplexing of serially arranged gratings can be achieved, see A. D. Kersey, T. A. Berkoff, and W. W. Morey, “Multiplexed fiber Bragg grating strain-sensor system with a fiber Fabry-Perot wavelength filter,” Opt. Lett. 18, 1370-1372 (1993). Alternatively, a scanning laser can be employed instead of the broadband source and tunable filter to provide identical functionality. These schemes require precise control of the tunable spectral components to achieve satisfactory peak detection resolution and accuracy. However, high levels of performance are difficult to achieve in practical systems due to inherent nonlinearities, drifts, and electronic noise. These effects are particularly difficult to control at higher scanning frequencies. Therefore, conventional interrogation systems possess a trade-off between their extensibility (number of sensors the scheme can interrogate) and the quality of the measurement (accuracy, resolution, and rate of measurement).
An alternative, highly precise peak detection technique employing interferometry can be used, as described by A. D. Kersey, T. A. Berkoff, and W. W. Morey, in “High-resolution fibre-grating based strain sensor with interferometric wavelength-shift detection,” Elec. Lett. 28 (3), 236-238 (1992). However, this technique is difficult to combine with existing multiplexing methods. One example of such a combined system is reported by Todd et al in “A novel Bragg grating sensor interrogation system utilizing a scanning filter, a Mach-Zehnder interferometer and a 3×3 coupler,” Meas. Sci. Technol. 12, 771-777 (2001). However, this has a limitation of the interrogation speed due to the use of a tunable filter. Additionally, the high cost of tunable filters prevents this technology from being widely adopted.
U.S. Pat. No. 5,680,489 describes an optical interrogation system that uses fibre Bragg gratings, at least one interferometer and optical multiplexing techniques, such as differentiate-cross-multiplying and time-division multiplexing. The system uses a conventional interferometer to interrogate each sensor in parallel. Signal processing and control systems are used to modulate one arm of the interferometer to apply a periodic phase signal at the output. A problem with the system of U.S. Pat. No. 5,680,489 is that the speed of multiplexing and the measurement bandwidth are limited by the requirement for active interferometer path modulation.
US 2002/0041722 describes an optical sensing device containing fiber Bragg gratings, a scanning Fabry-Perot bandpass filter, an interferometer and multiple photodetectors. The scanning Fabry-Perot (SFP) bandpass filter is used to wavelength-multiplex multiple gratings in a single fiber, and an unbalanced Mach-Zehnder fibre interferometer made with a 3×3 coupler is used to detect strain-induced wavelength shifts. The use of a tunable filter limits the achievable interrogation rate and wastes valuable time during the scanning operation.